The invention relates to an electromagnetic hysteresis unit.
By electromagnetic hysteresis units will be understood hereinafter as hysteresis brakes and hysteresis clutches. The method of operation of the hysteresis units is based on a magnetic action force of poles that attract each other in the synchronous running and on a constant magnetic reversal of a magnetically, semi-hard material, namely, of a hysteresis ring in the slip operation.
Unlike eddy-current clutches and brakes in hysteresis units, which are based on a different physical principle, the transmissible torque is, to a great extent, independent of the slip rotational speed.
The best known design of such hysteresis units consists of a magnet body with one exciting coil each having an outer and inner pole ring with axially aligned superposed soft iron poles in the same number and spacing, wherein the outer poles are disposed offset in peripheral direction relative to the inner poles in the stationary state or during synchronous running by half a spacing and have an opposite polarization. In the radial intermediate space of the pole rings, the hysteresis ring can rotate as a thin-walled, bell-shaped part without contact.
When magnet coils are traversed by current, a substantially radially oriented magnetic field generates between the poles of opposite polarity. But the pole off-set produces an alternatively tangential reorientation of the magnetic flow in the hysteresis ring and thus a permanent reverse magnetization of all elementary magnets when the hysteresis ring rotates relative to the magnetic body. Therefrom results a torque which depends only on the exciter flow. It can be regulated and controlled by adequately changing the exciting current. Such hysteresis units are known as clutch, e.g. from U.S. Pat. No. 2,488,827. Here the hysteresis ring is disposed radially between two parts of a rotatable magnet body which parts are connected by a disc of non-magnetizable material.
From DE 197 05 290 A1 is further known a hysteresis brake in which a hysteresis ring surrounds a closed magnetic ring of permanent magnetic material whose surface facing the hysteresis ring is provided with a plurality of poles embedded on the periphery and having alternatively opposite polarity. The magnetic ring is segmentally radially magnetized through and connected with a soft iron magnet body. The hysteresis ring rotates in an annular air gap between the magnet ring and an adjusting ring with slight radial play relative to the magnetic ring. The transmissible torque can be adjusted by an axial displacement of the adjusting ring.
The hysteresis ring is, in general, made of a material having small wall thickness and connected with a rotating part. Opposite to this, the parts provided with a large mass, such as magnet coils, magnet body, etc., are connected with the housing. In the case of a hysteresis clutch, one part of the magnet body is formed by a rotor and a magnetic-flux guiding disc connected therewith which rotates with slight play relative to the magnet body.
Because of the air gap between the hysteresis ring and the pole rings, the torque is contactlessly transmitted. Brakes produce both a brake torque in slip operation and a retaining torque in stationary state so that the decelerated part can also be kept in a decelerated position. Hysteresis clutches transmit torques both in synchronous running during which the coupled parts have the same rotational speed and in slip operation during which the parts to be coupled still have rotational speed difference. The transmissible torques depend only on the current in the exciting coil and can be continuously adjusted up to an admissible maximum value based on the type.
The power loss resulting in the slip operation heats the thin-walled hysteresis ring very quickly. The heat can be removed only very deficiently, via the small material thickness of the hysteresis ring, to the adjoining parts in order to be eliminated therefrom by further heat conduction and convection. The admissible permanent slip power on one side and the briefly removable slip work on the other are thus very limited.
Such hysteresis units are used, among others, for traction regulation for the processing of drawn endless products like wire, cable, rope, sheets, paper, threads, etc. They are also used for brake torque regulating systems and for a load simulation such as for test stands, ergometers, etc.
The problem on which the invention is based is to improve in the slip operation the brief and also the permanent thermal load of a hysteresis unit.
According to the invention the peripheral surfaces of the north poles and south poles lie on the same circle, the center of which lies on the axis of rotation. They also lie opposite the same peripheral surface of the hysteresis ring. Thereby the hysteresis ring is able to rotate on one peripheral surface at short distance from the magnets while on the other peripheral surface it is embedded in a rotor made of material having good heat conductivity which can also have cooling devices such as in the form of cooling ribs. It is thus ensured that the heat accumulated be thoroughly removed and that great slip torques can be transmitted for a long time.
The hysteresis ring conveniently surrounds the north poles and the south poles so that it lies with the adjoining rotor parts on the outer periphery of the hysteresis unit. Thereby result, on one hand, large heat radiation surfaces and, on the other, the rotor generates in this area itself a great air movement which favors the convention. In the peripheral area of the rotor cooling ribs are conveniently disposed which can be aligned both axially and in peripheral direction and can be interrupted by slots.
In one development of the invention, the poles are formed by pole fingers which, departing from axial front walls of the magnet body, are fitted upon each other and have between them a larger distance than from the hysteresis ring so that the magnetic flux leads from a north pole to a south pole via the hysteresis ring. The pole fingers can here advantageously overlap in peripheral direction.
The pole fingers, which extend substantially axially, taper toward their free end in axial direction and/or in peripheral direction. Thereby material and weight are spared and a good magnetic flux obtained. In particular, the tapering in peripheral direction produces a very uniform distribution of the magnetic flux between adjacent pole fingers so that the hysteresis ring uniformly absorbs energy on its breadth and local temperature peaks are prevented.
The magnet body can be easily produced with its pole fingers when it is divided in a radial plane, each separate part of the magnet body receiving the pole finger of one polarity. A centering ring reciprocally centers the two parts of the magnet body which are usually interconnected with screws,.
To keep the rotating masses small, it is advantageous that the magnet body with the magnet coil be disposed fastened on the housing and to use a free space between the pole fingers for current supply. In combination with a pot-shaped rotor open on one side, current can be supplied without an expensive sliding ring arrangement subject to wear.
To increase the stability of the pole fingers, the same as to prevent vibrations and flow noises, it is convenient that the pole fingers be interconnected by a non-magnetizable material. If the material, preferably brass, has good heat conductivity, it can, at the same time, serve uniformly to distribute the accumulated heat and remove it to the outside. The material can be advantageously introduced as filling composition in the intermediate spaces between the pole fingers or be formed by a connecting ring upon which the pole fingers are shrunk.
When the hysteresis unit is designed as clutch, an outer part of the first magnet body is conveniently separated by an annular gap and connected with the second magnetic body via the non-magnetizable material. The rest of the first magnet body with the coil are mounted fastened on the housing while the second magnet body sits on a part to be coupled and is rotatably supported with a small gap relative to the first magnet body. A second part to be coupled is non-rotatably connected with the hysteresis unit which carries the hysteresis ring.